Background: The discovery of fibroblast growth factor 23 (FGF23) provides a new conceptual framework that improves our understanding of the pathogenesis of post-transplant bone disease. Excess FGF23 is produced in the early post-transplant period; levels return to normal in the months following transplant. However, few manuscripts discuss FGF23 levels in stable long-term renal transplant recipients.
Methods: We performed a cross-sectional observational study of 279 maintenance kidney recipients with chronic kidney disease (CKD) Stages 1-4 and stable allograft function who had received their transplant at least 12 months previously. We calculated the estimated GFR (eGFR) using the MDRD4 equation.
Results: FGF23, parathyroid hormone (PTH) and phosphorus values were higher in more advanced stages, while the serum calcitriol levels and the phosphate reabsorption rate were lower. A significant inverse correlation was found between eGFR and FGF23 (r = -0.487; P < 0.001), PTH (r = -0.444; P < 0.001), serum phosphate levels (r = -0.315; P < 0.001) and fractional excretion of magnesium (r = -0.503; P < 0.001). Multivariable analysis showed that increased time on corticosteroids (P < 0.001), PTH (P < 0.001), serum phosphate (P = 0.003), decreased serum calcitriol (P = 0.049) and estimated glomerular filtration (P = 0.003) rate were associated with high FGF23 levels. In contrast with pre-transplant patients and first year post-transplant patients, higher FGF23 values were not correlated with increased phosphate excretion. An elevated phosphate reabsorption rate was associated with decreased PTH (P < 0.001) and calciuria (P = 0.028) and increased serum calcitriol (P = 0.009), plasma bicarbonate (P = 0.024) and estimated glomerular filtration (P = 0.003).
Conclusions: Serum FGF23 concentrations remain increased in long-term kidney graft recipients, even in the early stages of CKD. It remains to be seen whether measures aimed at reducing serum levels of PTH and phosphate and/or corticosteroid doses might help to lower serum FGF23 and whether this will improve kidney recipient outcomes.